This invention relates to vapor desolventizing, and more particular, to a process for the vapor desolventizing of spent oleaginous seed materials extracted with aqueous alcohol solutions.
Oleaginous seed materials, such as soybeans, cottonseeds, peanuts, sesame seeds, and sunflower seeds contain high concentrations of nutritious proteins. Other constituents are fiber, oil and carbohydrates. Since the carbohydrates are often associated with characteristic undesirable flavors and since they are also the cause of flatulence when the untreated seeds are used in a human diet, it is known to extract flaked seeds first with a solvent, such as hexane to remove the oil content and then to extract the defatted flakes with a carbohydrate-selective solvent, to leave as the residue a protein concentrate.
Protein concentrate is a product made by selectively extracting carbohydrate from a deoiled seed to leave as residue a product of high protein content. More specifically, soybean protein concentrate is made by extracting de-oiled, undenatured, "white" soybean flakes, the residue containing by commercial definition at least 70% protein. The de-oiled so-called "white" soybean flakes used are made by extracting oil from dehulled flakes with hexane followed by desolventizing the extracted flakes, i.e. removing solvent from the de-oiled flakes by heating without overheating so as to minimize denaturation of the protein, as measured by protein solubility or dispersibility. In the process with which the present invention is concerned, white flakes are further extracted with aqueous alcohols such as methanol, ethanol or isopropanol to make soy protein concentrate.
It is known that carbohydrates can be extracted from de-oiled flakes with little extraction of protein, using strong alcohol solutions. Ethanol is a preferred alcohol because there is no objection to a food product which may contain traces thereof. Although the process of the present invention is described with reference to aqueous ethanol as the solvent, and to soybeans, it is to be understood that the invention is applicable, whatever the aqueous organic solvent, and whatever the oilseed being treated.
The concentration of ethanol in aqueous solution which may be practically used for selective extraction of carbohydrates ranges from between 50 to 75% by weight. At concentrations below 50%, the flakes become wetted and fragile. At concentrations above 75%, a product containing 70% protein cannot be realized since the carbohydrates which need to be extracted are not sufficiently soluble. If denaturation of protein in the product is not of consequence, it is advantageous to use for extraction a concentration of ethanol at the lower end of the range at well above ambient temperature, because low concentration and high temperature promote carbohydrate extraction. However, both low concentrations and high temperatures cause rapid denaturation of proteins. If protein solubility is to be preserved, the concentration of ethanol in the aqueous organic solvent must be above about 65% and extraction temperature not higher than about 95.degree.F. (the lowest temperature that can be used in practice without resorting to refrigeration of cooling water).
Protein denaturation is measured by such standard tests as Protein Dispersibility Index (PDI), Nitrogen Solubility Index (NSI) and indirectly by Water Absorption Index (WAI). PDI is measured by dispersing flakes in water, then separating by centrifuging the flakes from the aqueous phase. PDI is the percentage of the nitrogen in the flakes found in the aqueous phase.
The present invention is concerned with the recovery of aqueous organic solvent from extracted spent flakes by heating (desolventizing) with minimal loss of protein dispersibility. It is known to recover solvent by heating either by conveying the spent flakes through a steam-jacketed conveyor, or by bringing them into contact with superheated solvent vapor. The latter method is exemplified by the flash desolventizing system described in U.S. Pat. No. 3,367,034, with the vapor desolventizing system being described in U.S. Pat. No. 2,571,143. Desolventizing with super-heated solvent vapor is particularly applicable when the dispersibility of the protein is to be preserved by minimizing retention time and temperature during desolventizing. Conventional flash desolventizing with superheated vapor has been applied to desolventizing alcohol-extracted flakes at the Northern Regional Research Laboratory of the U.S. Department of Agriculture (Mustakas et. al., Journal of the American Oil Chemists' Society 38, 473 - 8 (1961) and 39, 222 - 6 (1961). Under conditions at which it might have been expected that protein dispersibility would have been retained, protein was practically completely denatured in the desolventizing step when the solvents were 70% methanol and 70% ethanol. There was little or no protein denaturation when the solvents were absolute ethanol, 95% ethanol, and 91% isopropanol.
A complete process for producing protein concentrate includes an extraction step, a spent flakes desolventizing step, and a miscella evaporation step. Although the miscella evaporation step, per se, is not part of the present invention, the present invention does require that miscella evaporation include a rectification step to produce a vapor stronger in alcohol concentration as compared to the alcohol concentration of the aqueous alcohol solvent.